Morphological, Anatomical and Physiological Responses Related to Differential Shoot vs. Root Growth Inhibition at Low Temperature in Spring and Winter Wheat

Several morphological, anatomical and physiological changesand their relationship with differential root vs. shoot growthinhibition at low temperature (5°C) were studied in springand winter wheat cultivars. Root:shoot ratios, expressed eitheras a function of root and shoot fresh weight or as a functionof root and leaf areas, increased at low temperature and thisincrement was more pronounced in spring cultivars than in winterones. Although winter cultivars developed relatively smallerroot systems at 5°C, this characteristic was counterbalancedby a lower stomatal frequency and increased thickness of epidermalcell walls in leaves unfolded at this temperature, relativeto spring cultivars. Likewise, at 5°C a decrease in theosmotic potential of shoots and roots was observed in parallelwith sugar accumulation; this decrease was more marked in wintercultivars. These results indicate a differential morpho-anatomicaland physiological plasticity of winter and spring cultivarsduring development at low temperature. The possible associationbetween these changes and plant water economy at low temperaturesis discussed. Copyright 2001 Annals of Botany Company Spring wheat, winter wheat, Triticum aestivum, low temperature, root:shoot ratio, root surface area, stomatal frequency, osmotic potential     

Effect of Root and Shoot Meristem Temperature on Shoot to Root Dry Matter Partitioning and the Internal Concentrations of Nitrogen and Carbohydrates in Maize and Wheat

Maize (Zea mays L.) and spring wheat (Triticum aestivum L.)were grown in nutrient solution at uniformly high air temperature(20 °C), but different root zone temperatures (RZT 20, 16,12 °C). To manipulate the ratio of shoot activity to rootactivity, the plants were grown with their shoot base includingthe apical meristem either above (i.e. at 20 °C) or withinthe nutrient solution (i.e. at 20, 16 or 12 °C). In wheat, the ratio of shoot:root dry matter partitioning decreasedat low RZT, whereas the opposite was true for maize. In bothspecies, dry matter partitioning to the shoot was one-sidedlyincreased when the shoot base temperature, and thus shoot activity,were increased at low RZT. The concentrations of non-structuralcarbohydrates (NSC) in the shoots and roots were higher at lowin comparison to high RZT in both species, irrespective of theshoot base temperature. The concentrations of nitrogen (N) inthe shoot and root fresh matter also increased at low RZT withthe exception of maize grown at 12 °C RZT and 20 °Cshoot base temperature. The ratio of NSC:N was increased inboth species at low RZT. However this ratio was negatively correlatedwith the ratio of shoot:root dry matter partitioning in wheat,but positively correlated in maize. It is suggested that dry matter partitioning between shoot androots at low RZT is not causally related to the internal nitrogenor carbohydrate status of the plants. Furthermore, balancedactivity between shoot and roots is maintained by adaptationsin specific shoot and root activity, rather than by an alteredratio of biomass allocation between shoot and roots.Copyright1994, 1999 Academic Press Wheat, Triticum aestivum, maize, Zea mays, root temperature, shoot meristem temperature, biomass allocation, shoot:root ratio, carbohydrate status, nitrogen status, functional equilibrium     

Effects of Low Temperature on Growth and Nutrient Accumulation in Rye (Secale cereale) and Wheat (Triticum aestivum)

Rye (Secale cereale cv. Rheidol) and wheat (Triticum aestivumcv. Mardler) were grown at shoot/root temperatures of 20/20°C (warm grown, WG plants), 8/8 °C (cold grown, CG plants)and 20/8 °C (differential grown, DG plants). Plants fromcontrasting growth temperature regimes were standardized andcompared using a developmental timescale based on accumulatedthermal time (°C d) at the shoot meristem. Accumulationof dry matter, nitrogen and potassium were exponential overthe time period studied (150–550 °C d). In rye, therates of plant dry matter and f. wt accumulation were linearlyrelated to the temperature of the shoot meristem. However, inwheat, although the rates of plant dry matter and f. wt accumulationwere temperature dependent, the linear relationship with shootmeristem temperature was weaker than in rye. The shoot/rootratio of rye was stable irrespective of growth temperature treatment,but the shoot/root ratio of wheat varied with growth temperaturetreatment. The shoot/root ratio of DG wheat was 50% greaterthan WG wheat. In both cereals, nutrient concentrations anddry matter content tended to be greater in organs exposed directlyto low temperatures. The mean specific absorption rates of nutrientswere calculated for the whole period studied for each species/temperaturecombination and were positively correlated with both plant shoot/rootratio and relative growth rate. The data suggest that nutrientuptake rates were influenced primarily by plant demand, withno indication of specific nutrient limitations at low temperatures. Nutrient accumulation, relative growth rate (RGR), rye, Secale cereale cv. Rheidol, temperature, thermal time, Triticum aestivum cv. Mardler, wheat     

Root Zone Temperature Effects on Growth and Nitrate Absorption in Rape (Brassica napus cv. Emerald)

Effects of root temperatures, ranging from 10–35 °C, on growth and nitrate inflow of fodder rape seedlings (cv.Emerald) were examined. These were cultured in solution, withtheir shoots held at 25 ° C. Nitrate inflow (uptake rateper unit root length) was little affected over the temperaturerange 10–30 ° C, although enhanced values were foundat 35 ° C. Nitrate absorption by roots at 10-30 ° Cdepleted solution concentrations to an apparent minimum of approximately6.0 µM NO₃^–. Relative growth rates were highestwith root temperatures of 25 ° C and 30 °C, and thesewere associated with the greatest nitrate depletion rates fromsolution. Root: shoot weight ratios were also greatest at 25°C and 30 °C. At 10 °C and 35 °C a relativelylarge shoot on a small root maintained nitrate inflow in spiteof the plants' slow growth rate. The nitrogen concentrationin the shoots was little affected by root temperature. Slowgrowth at a root temperature of 10 °C was not associatedwith a shortage of nitrogen in the shoots. The principal influenceof temperature appears to be on extension and differentiationof root tissues, possibly through effects on carbohydrate supplyto root meristems.     

Cold-Induced Alterations in Plasma Membrane Proteins That are Specifically Related to the Development of Freezing Tolerance in Cold-Hardy Winter Wheat

The objective of this study was to identify plasma membraneproteins that are specifically induced by cold acclimation inwheat (Triticum aestivum L.). Two cultivars with a marked differencein the genetic ability to cold-acclimate, namely, spring wheat(cv. Chinese Spring) and winter wheat (cv. Norstar), were usedas the experimental material. After four weeks of growth ina cold chamber, the freezing tolerance in the shoots of winterwheat increased to –18°C, whereas it increased onlyto –8°C in the shoots of spring wheat. In the caseof roots from both cultivars, freezing tolerance increased onlyslightly after the growth in the cold environment. Cold acclimationinduced remarkable changes in the electrophoretic patterns ofplasma membrane proteins which depended on both the cultivarand the tissue examined. Levels of polypeptides with molecularmasses from 22 to 31 kDa decreased in both the root and shootplasma membranes from both cultivars. Among these polypeptides,levels of those of 28 and 26 kDa decreased abruptly after oneweek of cold acclimation. By contrast, levels of polypeptidesof 89, 83, 52, 23, 18 and 17 kDa increased specifically in theshoots of winter wheat. The increases in the levels of the 23-,18- and 17-kDa polypeptides were proportional to the developmentof freezing tolerance. Freeze-fracture electron microscopy ofplasma membranes from shoot cells revealed that the number ofintramembrane particles on the fracture faces decreased markedlyin winter wheat after cold acclimation, but to a lesser extentin spring wheat. These results suggest that the plasma membranesmight undergo molecular reorganization during cold acclimation. ¹Contribution no. 3709 from the Institute of Low TemperatureScience, Hokkaido University.     

Correlative Inhibition in the Shoot of Agropyron repens ( L.) Beauv

Correlative inhibition was investigated in plants of Agropyronrepens at two temperatures. Reciprocal inhibition ocrurred betweenthe main shoot apex and the outgrowing axillary shoots, withthe balance of inhibition varying with temperature. Apical dominancewas stronger at 10 °C than at 20 °C , but even at 10°C release of apical dominance by decapitation had onlyminor effects on the timing of outgrowth, growth pattern andrate of dry weight aocumulation of the axillary shoots. Dominanceof the main shoot apex by the axillary shoots was stronger at20 °C than at 10 °C. Removal of axillary buds preventeddecline in size and activity of the main shoot apex ard resultedin increased rates of primordium initiation, leaf emergenceand dry weight accumulation in the main shoot. It is suggestedthat a system of reciprocal dominance provides a mechanism formaintaining the characteristic habit of the grass plant andlimits growth in height of vegetative shoots. Agropyron repens (L.) Beauv, couch grass, correlative inhibition, apical dominance, shoot, apex     

Low Temperature-induced Accumulation of Protein is Sustained both in Root Meristems and in Callus in Winter Wheat but not in Spring Wheat

Plant growth was assessed and cellular protein per nuclear DNAamount measured in root meristems and in callus derived fromembryos of a spring (‘Katepwa’) and a winter variety(‘Beaver’) of allohexaploid wheat exposed to lowtemperature treatment. The data obtained were used to test whetherthese genetically distinct varieties of wheat responded differentiallyto cold treatment. Seedlings were grown for 14 d at 20°Cand then transferred to 4°C for 14 d before returning themto 20°C, or else were maintained continuously at 20°C.In winter wheat, root growth at 4°C was significantly greaterover the first 7 d following transfer to 4°C compared withplants retained at 20°C, whereas in spring wheat it wasreduced at 4°C. The pattern of accumulation of cellularprotein for both root meristem cells and in callus cells wasalso generally enhanced at 4°C compared with 20°C inwinter wheat but not in spring wheat. Thus, clear inter-varietaldifferences were established both for dry weight accumulationand cellular protein, and the callus data clearly show thatthe low-temperature-induced accumulation of protein is a cellularphenomenon not necessarily linked to development. The extentto which cold-shock proteins are a component of this low temperature-inducedincrease in cellular protein is discussed. Copyright 2000 Annalsof Botany Company Protein, spring wheat, temperature, tissue culture, Triticum aestivum, winter wheat     

Sugar Accumulation in Barley and Rice Grown in Solutions with low Concentrations of Oxygen

Barley and rice, at the early tillering stage, were grown inaerated nutrient solutions (> 7 mg O₂ l^–1) and transferredto solutions of low O₂ concentrations (< 0.5 mg l ^–1). For barley, low O₂ concentrations during the first 5 days severelyinhibited growth of seminal roots had less effect on nodal roots,and did not reduce shoot growth. Longer exposure to low O₂ concentrationsreduced shoot as well as root growth. Sugar concentrations inroots and shoots increased within 7 h after transfer of plantsto low O₂ concentrations. After 5 days at low O₂ concentrationssugar concentrations were very high in fast growing nodal rootsand in shoots, as well as in the slower growing seminal roots. In rice, low O₂ concentrations increased sugar levels of rootsduring summer, but not during winter. In summer, the highersugar levels at low O₂ concentrations persisted throughout adiurnal cycle. In root apices, sugar concentrations were increasedby low O₂ concentrations, even though the experiment was donein winter and the bulk of the root system showed no differencein sugar levels. The data indicate that sugar accumulation, at low O₂ concentrations,is caused by reduced growth and also that even apices of rootsgrown at low O₂ concentrations have sufficient substrates forrespiration. Hordeum vulgare L, barley, Oryza sativa L, rice, sugar accumulation, oxygen concentration     

Does the lack of vernalization requirement interfere with winter survival of oilseed rape plants?

Recent studies (Rapacz 1999) have shown that cultivars of spring-type oilseed rape are able to cold-acclimate to the level comparable with winter cultivars, but only after prehardening which results both in the increase of photosynthetic activity and in growth cessation. It is commonly known that under field conditions spring-type cultivars could not survive winter. Present studies were undertaken to explain the reasons for low winter hardiness of spring type rape plants. Six cultivars of spring and two of winter rape were sown in the open-air vegetation room at the end of August. The obtained results indicate that the degree of frost damage in spring-type plants increased in the course of winter and this increase was parallel to elongation of generative shoots observed after periods of warming. Each spring cultivar was completely killed by frost just after its generative shoot reached 15–20 cm, irrespective of its frost resistance level, determined previously under laboratory conditions. In the case of winter cultivars survival rate was consistent with laboratory-estimated frost resistance. It is suggested that spring rape could not survive winter because of its limited ability to prevent shoot elongation during winter at temperatures slightly above 0 °C. It was also found that less efficient photosynthetic electron transport in autumn was observed in these spring cultivars in which the elongation of generative shoots was observed already during the first warm break in winter.     

Leaf Extension in Zea mays: I. LEAF EXTENSION AND WATER POTENTIAL IN RELATION TO ROOT-ZONE AND AIR TEMPERATURES

The temperature of the roots and shoots of Zea mays plants werevaried independently of each other and the rates of leaf extensionand leaf water potentials were measured. Restrictions of leafextension occurred when root temperatures were lowered from35 to 0 °C, but leaf water potentials were lowered onlyat root temperatures below 5 °C. Similar changes in ratesof leaf extension were measured at air temperatures from 30to 5 °. Between 30 and 35 °C air temperature, in anunsaturated atmosphere, restrictions of leaf extension wereassociated with low leaf water potentials. It was concluded that, at root temperatures 5 to 35 °C,and shoot temperatures 5 to 30 °C, water stress was notthe main factor restricting the extension of Zea mays leaves.     

Analysis of Dry Matter Partitioning in Dactylis glomerata during Vegetative Growth Using a Carbon Budget Model

The dry matter partitioning in vegetative plants of Dactylisglomerata was studied from experiments performed in controlledenvironments. Plants were grown hydroponically in growth chambers,at two constant temperatures (17 and 25 °C). In both experimentsthe root fraction decreased regularly with time, an effect thatwas more accentuated in the higher temperature regime. In orderto explain the change in dry matter partitioning, the experimentalshoot and root growth were analysed using a carbon budget modelwhich includes shoot and root maintenance requirements. Themodel predicts a relationship between the root specific growthrate and the product of shoot specific growth rate and shootto root dry weight ratio. In the range of experimental accuracy,this relationship was found to be linear at both temperatures,which should indicate that the partitioning coefficients andthe root maintenance coefficient remained constant during vegetativegrowth. The effect of temperature on the value of these coefficientscan be specified from a linear regression analysis. Between17 and 25 °C, the root maintenance coefficient increasedby about a factor of two, whereas the partitioning coefficientsdid not vary significantly. On the basis of these results, itwas shown that the decrease in root fraction during vegetativegrowth should be mainly attributed to the decrease in net specificactivity of shoots.Copyright 1994, 1999 Academic Press Dactylis glomerata L., vegetative growth, model, partitioning, root:shoot ratio, shoot specific activity, maintenance requirements     

The Effect of Temperature on Growth and Development of Echinochloa Millets

Accumulation of dry weight and leaf plus stem area were measuredin Echinochloa utilis and E. frumentacea grown at temperatureregimes from 15/10°C to 33/28°C (day/night). Tilleringand height were recorded in addition to leaf number which wassubsequently used as a developmental index. In both species shoot dry weight increased with temperatureup to 33/28°C; the increase in relative growth rate (RGR)was negligible above 27/22°C. Below 27/22°C the RGRof E. frumentacea decreased sharply and at 15/10°C it madeno effective growth. At low temperatures the RGR of E. frumentaceawas lower than that of E. utilis due to slow leaf area expansion,and in particular smaller individual leaves. E. frumentaceatillered more than E. utilis. Plant development was retardedat low temperatures but was not as responsive to temperatureas dry weight and leaf area. The different responses to temperatureof the two species were described in equations suitable forinclusion in predictive growth models. Echinochloa spp., millet, growth, development, temperature, relative growth rate     

Low temperature differentially affects tillering in spring and winter wheat in association with changes in plant carbon status

The higher tillering potential of winter wheat cultivars as compared to spring ones has been attributed to an earlier tillering onset in the former. Tillering in grasses is known to depend on temperature and light environment, and may be mediated by plant C status. The aim of this work was to elucidate whether differences in carbohydrate accumulation between wheat types at low temperature may contribute to differences in tillering potential. A set of experiments with contrasting temperatures was conducted in both field enclosures (high irradiance, three experiments) and growth chambers (low irradiance, two experiments). The relative tiller production rate (RTR) was the highest in winter cultivars grown in cool field enclosures, and the lowest in spring cultivars in growth chambers, either cool or warm. Plant C status was inversely related to temperature the response being more pronounced in winter cultivars. Components of RTR, site filling and phyllochron, responded differently to environment and plant C status. Phyllochron increased with temperature, and was inversely correlated to sugar concentration irrespective of cultivar type. Site filling increased with irradiance in both cultivar types while sugar concentration contributed additively to its promotion solely in winter cultivars. Nevertheless, variation in site filling was larger in percentage than variation in phyllochron (200% and 41%, respectively, between most contrasting treatments). Thus, differences in tillering potential between winter and spring wheats may be attributed not only to the earlier tillering onset in the former but also to their differential response to environment and C status.     

Developmental Physiology of Sugar-beet: IV. EFFECTS OF GROWTH SUBSTANCES AND DIFFERENTIAL ROOT AND SHOOT TEMPERATURES ON SUBSEQUENT GROWTH

The effects of three growth substances, viz. indol-3yl-aceticacid (IAA), gibberellic acid (GA₃), and kinetin (KIN), and differentialshoot and root temperatures on growth of sugar-beet (Beta vulgarisL.) plants have been studied. IAA, GA₃, and KIN were applied in aqueous lanolin at differentconcentrations (50 ppm to 5000 ppm) to decapitated sugar-beetplants at the eight-leaf stage, one group also having alternateleaves removed. The growth substances significantly increasedthe dry weights of the plants when all the leaves were present,which was mainly explained by the large increase in roots. Thegrowth substances probably stimulated cambial activity and hencethe mobilization of substrates resulting in a bigger root whena relatively large leaf area existed. The failure of the plantsto respond to treatments following the removal of alternateleaves suggests that under such conditions the growth substanceshave hardly any major effect on the production of substrates;rather they influence growth by regulating the movement of substratesby altering the ‘sink strength’ if the supply ofsubstrates is not limiting. It could also be that the rootsproduce sufficient growth substances to maintain half the leavesat maximum expansion and maximum photosynthesis. Treatment withgrowth substances would therefore have little effect. When allthe leaves were present, they are limited by insufficient growthsubstances. All combinations of root and shoot temperatures of 17 and 25°C were imposed on plants decapitated at the eight-leafstage, one group also having each alternate leaf removed. Leaf8 expanded most at shoot and root temperature of 25 °C whereasother leaves had the largest areas at shoot and root temperatureof 17 °. When all the leaves were present root growth wasmaximal at shoot temperature of 17°C and root temperatureof 25 °C, but when alternate leaves were removed maximumroot growth occurred at shoot and root temperatures of 25 °C.Generally, a higher concentration of soluble carbohydrates wasfound in the roots and leaves when either the shoot or rootor both were kept at 17 °C. Concentrations of nitrogen,phosphorus, and potassium in different organs were less at 17°C than at higher shoot or root temperatures and decreasedwith age.     

Critical Concentration of Tissue 9

Solution culture experiments with spring wheat (Triticum aestivumcv. Neepawa) showed that the critical phosphorus concentrationin the shoot (CPC) was lower when root temperatures were optimalfor growth. Root temperatures of 10, 15, 20 and 25°C wereused, the optimal was 25°C. The plants were sampled at definedstages, either at 4·0 g fr. wt. or with 6 mature leavesand the phosphorus (P) status of the plants at the samplingtime was evaluated by observing the responses of other plantsto an additional supply of P. These two precautions were criticalto the interpretation of the data and may explain the discrepancyin the literature about the relationship between plant growthrate and the CPC. Key words: Relative growth rate, Wheat     

Effects of CO2 Enrichment at Different Irradiances on Growth and Yield of Wheat: I. EFFECTS OF CULTIVAR AND OF DURATION OF CO2 ENRICHMENT

Wheat, Triticwn aestivum L., the winter cultivars Hobbit andCappelle-Desprez, and the spring cultivars Sicco and KJeiber,were grown in normal air or air enriched with CO₂ either outdoorsin a glass-roofed cage or in controlled environment rooms. Inneither the winter nor the spring wheat was growth increaseddue to enrichment with CO₂ before anthesis. Enrichment of thetwo winter wheat cultivars increased shoot dry weight significantlyat 15 d after anthesis but produced no significant increasein grain yield. With the spring cultivars there was a significantincrease in shoot dry weight by 18 d after anthesis and thegrain yield was also larger due to an increase in grain size.Shoot weight increased because the stems were larger, and therewas a diversion of assimilate from grain growth to late tillerproduction. Root tissue comprised less than 20% of the totaldry matter at anthesis (for all cultivars); effects of CO₂ enrichmenton root growth appeared to be less important than effects onshoot and ear growth. Growth and yield responses to CO₂ enrichmentwere observed (for the spring cultivars) at irradiances of both250 and 635 µE m^–2 s^–1, but the effects weregreater at the lower irradiance. Key words: CO₂ enrichment, Wheat, Cultivar     

Primary Growth and Re-growth of the Tropical Tallgrass Hybrid Pennisetum at Different Temperatures

Dry weight of plant fractions, leaf area, leaf number and tillernumber were recorded throughout primary growth and two subsequentre-growths of hybrid Pennisetum (Pennisetum americanum x P.purpureum) at five temperature regimes from 15/10 °C to33/28 °C (day/night) in summer and winter. Seedling mortality occurred at 15/10 °C, whereas at allhigher temperatures seedlings survived and plants re-grew aftercutting at a height of 10 cm. Shoot weights increased with temperatureup to 33/28 °C when compared at a common chronological agebut showed no differences at a common developmental age. Thetemperature response was associated with increased top/rootratio and rate of leaf appearance; mean individual leaf areaand NAR did not increase beyond 27/22 °C. Shoot weight incrementsin primary growth were the same in winter and summer when expressedper unit of radiation, although leaf area per unit weight wassensitive to changes in radiation associated with differencesin daylength. The rate of shoot weight accumulation in regrowthwas greater than in primary growth because of rapid tilleringfollowing defoliation and an enhanced rate of leaf appearanceper tiller. Pennisetum hybrid, tallgrass, growth, regrowth temperature response     

Concurrent Rates of N2 Fixation, Nitrate and Ammonium Uptake by White Clover in Response to Growth at Different Root Temperatures

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown for 71 d in flowing nutrient solutions containingN as 10 mmol m_–3 NH₄NO₃, under artificial illumination,with shoots at 20/15°C day/night temperatures and root temperaturereduced decrementally from 20 to 5°C. Root temperatureswere then changed to 3, 7, 9, 11, 13, 17 or 25°C, and theacquisition of N by N₂ fixation, NH₄+ and NO₃^– uptakewas measured over 14 d. Shoot specific growth rates (d. wt)doubled with increasing temperature between 7 and 17°C,whilst root specific growth rates showed little response; shoot:root ratios increased with root temperature, and over time at11°C. Net uptake of total N per plant (N₂ fixation + NH₄++ NO₃^–) over 14 d increased three-fold between 3 and 17°C.The proportion contributed by N₂ fixation decreased with increasingtemperature from 51% at 5°C to 18% at 25°C. Uptake ofNH₄+ as a proportion of NH₄+ + NO₃^– uptake over 14 d variedlittle (55–62%) with root temperature between 3 and 25°C,although it increased with time at most temperatures. Mean ratesof total N uptake per unit shoot f. wt over 14 d changed littlebetween 9 and 25°C, but decreased progressively with temperaturebelow 9°C, due to the decline in the rates of NH₄+ and NO₃^–uptake, even though N₂ fixation increased. The results suggestthat N₂ fixation in the presence of sustained low concentrationsof NH₄+ and NO₄^– is less sensitive to low root temperaturethan are either NH₄+ or NO₃^– uptake systems. White clover, Trifolium repens L. cv. Huia, root temperature, nitrogen fixation, ammonium, nitrate     

Effect of respiratory homeostasis on plant growth in cultivars of wheat and rice

Some plants have the ability to maintain similar respiratory rates (measured at the growth temperature), even when grown at different temperatures, a phenomenon referred to as respiratory homeostasis. The underlying mechanisms and ecological importance of this respiratory homeostasis are not understood. In order to understand this, root respiration and plant growth were investigated in two wheat cultivars (Triticum aestivum L. cv. Stiletto and cv. Patterson) with a high degree of homeostasis, and in one wheat cultivar (T. aestivum L. cv. Brookton) and one rice cultivar (Oryza sativa L. cv. Amaroo) with a low degree of homeostasis. The degree of homeostasis (H) is defined as a quantitative value, which occurs between 0 (no acclimation) and 1 (full acclimation). These plants were grown hydroponically at constant 15 or 25 °C. A good correlation was observed between the rate of root respiration and the relative growth rates (RGR) of whole plant, shoot or root. The plants with high H showed a tendency to maintain their RGR, irrespective of growth temperature, whereas the plants with low H grown at 15 °C showed lower RGR than those grown at 25 °C. Among several parameters of growth analysis, variation in net assimilation rate per shoot mass (NAR_m) appeared to be responsible for the variation in RGR and rates of root respiration in the four cultivars. The plants with high H maintained their NAR_m at low growth temperature, but the plants with low H grown at 15 °C showed lower NAR_m than those grown at 25 °C. It is concluded that respiratory homeostasis in roots would help to maintain growth rate at low temperature due to a smaller decrease in net carbon gain at low temperature. Alternatively, growth rate per se may control the demand of respiratory ATP, root respiration rates and sink demands of photosynthesis. The contribution of nitrogen uptake to total respiratory costs was also estimated, and the effects of a nitrogen leak out of the roots and the efficiency of respiration on those costs are discussed.     

Growth and Partitioning in Pascopyrum smithii (C3) and Bouteloua gracilis(C4) as Influenced by Carbon Dioxide and Temperature

This study investigated how CO₂and temperature affect dry weight(d.wt) accumulation, total nonstructural carbohydrate (TNC)concentration, and partitioning of C and N among organs of twoimportant grasses of the shortgrass steppe,Pascopyrum smithiiRydb. (C₃) andBouteloua gracilis(H.B.K.) Lag. ex Steud. (C₄).Treatment combinations comprised two temperatures (20 and 35°C)at two concentrations of CO₂(380 and 750 µmol mol^-1),and two additional temperatures of 25 and 30°C at 750 µmolmol^-1CO₂. Plants were maintained under favourable nutrient andsoil moisture and harvested following 21, 35, and 49d of treatment.CO₂-induced growth enhancements were greatest at temperaturesconsidered favourable for growth of these grasses. Comparedto growth at 380 µmol mol^-1CO₂, final d.wt of CO₂-enrichedP.smithiiincreased 84% at 20°C, but only 4% at 35°C. Finald.wt ofB. graciliswas unaffected by CO₂at 20°C, but wasenhanced by 28% at 35°C. Root:shoot ratios remained relativelyconstant across CO₂levels, but increased inP. smithiiwith reductionin temperature. These partitioning results were adequately explainedby the theory of balanced root and shoot activity. Favourablegrowth temperatures led to CO₂-induced accumulations of TNCin leaves of both species, and in stems ofP. smithii, whichgenerally reflected responses of above-ground d.wt partitioningto CO₂. However, CO₂-induced decreases in plant tissue N concentrationswere more evident forP. smithii. Roots of CO₂-enrichedP. smithiihadgreater total N content at 20°C, an allocation of N below-groundthat may be an especially important adaptation for C₃plants.Tissue N contents ofB. graciliswere unaffected by CO₂. Resultssuggest CO₂enrichment may lead to reduced N requirements forgrowth in C₃plants and lower shoot N concentration, especiallyat favourable growth temperatures. Acclimation to CO₂; blue grama; Bouteloua gracilis ; carbohydrate; climate change; global change; grass; growth; growth temperature optima; nitrogen; N uptake; Pascopyrum smithii; western wheatgrass